Problem: 5 binders cost $8.35. Which equation would help determine the cost of 9 binders?
There are several equations that could help determine the cost, each with a slightly different approach. We know the cost of 5 binders. We want to know the cost of 9 binders. We can write the numbers of binders as a proportion: $\dfrac{5}{9}$ We know 5 binders costs $8.35. We can let $x$ represent the unknown cost of 9 binders. The proportion of these costs can be expressed as: $\dfrac{\$8.35}{x}$ The cost changes along with the number of binders purchased, and so the two proportions are equivalent. $\dfrac{5}{9} = \dfrac{\$8.35}{x}$